A variety of conventional electric motor-driven equipment systems are powered by variable frequency drives. In many applications, the variable frequency drive cannot be located adjacent to the equipment being driven. For example, an electric submersible pump used in hydrocarbon production may be separated from its associated variable frequency drive by thousands of feet of electric cable. Such wide separation generally demands the use of a filter on the output of the variable frequency drive. Further, a transformer is commonly used to step up the voltage in an effort to minimize the cable size. A long sought goal has been to increase a motor's power density (power per unit length) and equipment efficiency. One way to realize such higher equipment efficiencies is through the use of permanent magnet motors which achieve higher power density relative to a correspondingly sized induction motor. Relative to an induction motor, permanent magnet motors present especially difficult control problems. For example, efficient operation of the permanent magnet motor requires knowledge of the rotor position for proper synchronization with the stator currents. Commonly, a permanent magnet motor will use an angular position sensor, such as an encoder or resolver, in order to monitor the rotor position, at times referred to as the rotor angle. The rotor position data is transmitted from the motor to the variable frequency drive which uses the data to maintain proper synchronization of the rotor. However, for equipment deployed at a distance from the variable frequency drive and due to the frequently harsh conditions in which the equipment must operate, a physical sensor is un-attractive and additional options are needed because it is not practical to use a physical rotor position sensor under such circumstances.
An alternative to using a position feedback sensor located within the motor is to use detailed knowledge of the motor operating characteristics coupled with motor current and voltage measurements to estimate the rotor angle. These techniques are known as “sensorless” control techniques and in practice are typically very difficult to implement. Moreover, different models of variable frequency drives exhibit varying levels of ability to perform sensorless control. In some instances, a motor must be paired with a specific type of variable frequency drive as only certain variable frequency drives available in the marketplace may be compatible with a particular motor. As a result, motor performance may vary considerably depending on the variable frequency drive selected. As a practical matter, since the life expectancy of a motor deployed in a harsh environment may be considerably shorter than the life expectancy of the variable frequency drive which powers the motor, oil field operators would benefit if greater interchangeability could be achieved between different motors and available variable frequency drives. While the technical challenges discussed here apply to electric motors generally, they are particularly problematic when permanent magnet motors are involved because of the necessity of synchronizing stator currents with rotor position. Such uncertainty poses an obstacle for market adoption of equipment comprising permanent magnet motors despite the gains in operational efficiency conferred by the presence of the permanent magnet motor. Thus, there remains a need to further enhance the ease with which a variable frequency drive may be used to power a motor.